Calculating seeing

Folks - does anyone know if there is a way (eg with a CCD) to calculate seeing? I'm sure I read in one of the mags recently an article that mentioned seeing of a particular number (say, 1.7) It didn't go into detail as to how that number was arrived at but I got to thinking that it 'might' be possible to measure.

Any ideas?

Cheers

Dave McD

Folks - does anyone know if there is a way (eg with a CCD) to calculate seeing?

Yep.

Basically, you use the CCD to measure the size of the star. So you would plot the intensity of the values against the pixel location (a line of ccd pixels on one axis going through the center of the star).

On the plot note the peak intensity of the star.

Measure the width, in pixels, of the profile at Half of this intensity.

Now convert the number of pixels you have to arc seconds. Take a CCD image of two stars with a defined separation and work out the arcsecond/pixel.

This is defined as the Full width at Half Maximum(FWHM) (in arcseconds) which is a definition of seeing.

With good seeing you should get a nice spike, and so a small FWHM, under bad seeing the image will be bigger and less intense and the FWHM will be much broader FWHM.

The issue I have with this is that its not great for generally telling how good the seeing is to other people. It seems to be relativly specific to your scope and the calibration star. Perhaps I'm missing some details.

Cheers,
~Al

Did this as part of my final year physics project. Its really easy if your image analysis software can plot a graphy of intensity along a straight line. Then you just read off the hight of the top of the spike, measure half the hight and then measure the width of the spike at this value, that is your seeing (and as Al said it is called full width half max for reasons that I think should be obvious!)

Bart.

Thanks folks - appreaciate that info.

I wanted to know the method for calculating seeing so I can compare two images taken on different days. And I think, certainly the high end packages anyhow, have the ability to do the calculation for you.

Cheers again - you are truly righteous brothers!

Dave McD

Dave

I think you may be talking about the Pickering scale of seeing. It's rated out of 10 and is basically how many difractiion rings are visible in an image of a star. It's good in that you can image a star and gauge the scale direct from the image no guessing required- see Damian Peach's site for a full explanation.

Dave Gradwell

Folks - does anyone know if there is a way (eg with a CCD) to calculate seeing?

... This is defined as the Full width at Half Maximum(FWHM) (in arcseconds) which is a definition of seeing.

This will give you the FWHM of the entire optical system, of which the atmosphere (seeing) is just a part. In order to get the seeing, you will have to remove the effects of diffraction, and any optical imperfections in your telescope, from your measured FWHM.

In other words, you will tend to underestimate the quality of the seeing.